The present invention generally relates to ventilation systems for dead air spaces beneath structures, including (for example) cellars, basements and crawl spaces under houses or commercial structures. Such ventilation systems are typically designed to reduce or prevent foundation and subfloor wood rot, reduce build-up of molds, spores and the like, reduce the incidence of wet insulation, and reduce infestation by pests. Many structures are built with underlying dead air space which, for the sake of convenience, will be called crawl space for the purpose of this application. Although crawl space may be entirely enclosed and physically separated from the outside environment, often the structure foundation has vents or other portals directly or indirectly open to the outside atmosphere. In any event, due to the moisture content of soil and air, crawl spaces are often damp or humid, and accordingly susceptible to the problems mentioned above. Moisture tends to cause mold, mildew, rot and general corrosion of the structure foundation walls and subflooring; moisture also increases the likelihood of infestation by pests.
To avoid such problems, builders often install air vents in the foundation walls, intended to reduce interior moisture by replacing moist crawl space air with dryer outside air. There are a number of preexisting ventilators having a variety of features, such as opening and closing of vents or louvers to facilitate passive ventilation or the activation of fans for active ventilation, dependent upon temperature or crawl space humidity. Many patents relating to such ventilators are listed in U.S. Pat. No. 5,294,049 (the "'049 patent") and U.S. Pat. No. 5,253,804 (the "'804 patent").
The '049 issued to Trunkle, et al. claims a fan-powered foundation ventilator system having a louver housing that is selectively opened or closed depending upon the temperature of the crawl space air enclosed by the floors and the foundation walls of the building structure. The '804 patent issued to Sarazen, et al. claims a foundation ventilator apparatus including activated louvers or an electrical fan controlled by detected temperature or humidity; according to the disclosure in that patent, the opening and the closing of the louvers is independent of the electric fan operation. That patent essentially discloses a ventilator system having both active ventilation (fan dependent upon crawl space humidity) and passive ventilation (temperature-dependent louver opening) components.
One disadvantage of those inventions is that there is no mechanism for preventing ventilation by outside air that will actually be counterproductive by increasing the crawl space moisture. Such counterproductive conditions include time when outside air contains more absolute moisture than the crawl space air. Since the cited patents do not appreciate the differences between the absolute moisture content and the relative humidity of both outside and inside air, it is likely that such fan-powered ventilation may cause the exchange of outside air containing more moisture than its inside counterpart, leading to an increase in moisture within the crawl space and perhaps even condensation therein. As an example, assume it is 80.degree. F. outside with 50% relative humidity, whereas crawl space air is 65.degree. F. with 60% relative humidity; by standard calculations, the absolute moisture of the outside air is 13.37 mm Hg, whereas the absolute moisture of crawl space air is 9.29 mm Hg. Under these circumstances, if outside air moves into the crawl space and cools from 80.degree. F. to 65.degree. F., such replacement of crawl space air will result in a 44% increase in absolute moisture; accordingly, the relative humidity of the crawl space air will increase to 86%, using standard calculations.
As another example, assume it is 95.degree. F. outside with 45% relative humidity, whereas crawl space air is 65.degree. F. with 60% relative humidity; by standard calculations, the absolute moisture of the outside air is 18.98 mm Hg, whereas the absolute moisture of crawl space air is 9.29 mm Hg. Under these circumstances, if outside air moves into the crawl space and cools from 95.degree. F. to 65.degree. F., such replacement of crawl space air will result in a 104% increase in absolute moisture, and the relative humidity of the crawl space air will increase to 100%. Under such circumstances, condensation of water vapor will occur in the crawl space. These examples illustrate that the ventilation systems known in the field may result in the exchange of moisture-laden outside air for crawl space air having relatively less moisture.
U.S. Pat. No. 4,964,566 issued to Pugh, et al. discloses a ventilator system responsive to both the humidity level within the crawl space and to outside temperature. According to the disclosure of that patent, when outside temperatures are above freezing, and when the crawl space humidity rises above a predetermined level, a humidistat sends a signal to a solenoid to open the vent and cause a fan to exhaust the crawl space air. Like the two previously cited patents, this invention may cause the exchange of moisture-laden outside air for crawl space air having a relatively lower moisture content.
One primary object of the invention disclosed herein is to provide a ventilator that will cause the exchange of air only when it will lower the moisture in the crawl space. It is another object to provide a ventilator designed to reduce or prevent foundation and subfloor wood rot, and reduce the buildup of mold, spores and wet insulation. Another object includes providing a ventilator to better protect home inspectors, contractors and home owners from bacteria, molds, termiticides and other substances found in crawl spaces due to humidity. It is another object to provide a ventilator that saves heating and cooling energy costs, helps prevent pipes from freezing, and ventilates undesirable gases and chemicals.